With increased use and decreasing availability of petroleum supplies, coal gasification is currently becoming more attractive technically and economically as a versatile and clean way to convert the energy content of coal into electricity, hydrogen, and other high quality transportation fuels, as well as into high-value chemicals to meet specific market needs. Most importantly, in a time of unpredictable variations in the prices of electricity and fuels, gasification systems can provide a capability to operate on low-cost, widely-available coal reserves. Gasification may be one of the best ways to produce clean liquid fuels and chemical intermediates from coal as well as clean-burning hydrogen which also can be used to fuel power-generating turbines or used in the manufacture of a wide range of commercial products.
Four basic types of gasifiers are currently available for commercial use: counter-current bed, co-current bed, fluidized bed and entrained flow. In the counter-current fixed bed (“up draft”) gasifier the gasification agent (steam, oxygen and/or air) flows in counter-current configuration through a descending bed of the carbon-containing fuel with the ash removed dry or as a slag. The co-current bed gasifier is similar to the counter-current type, but the gasification agent gas flows downwards in the same direction as the fuel. In the fluidized bed reactor, the fuel is fluidized in the gasification agent. In the entrained flow gasifier a dry pulverized solid, an atomized liquid fuel or a fuel slurry is gasified with oxygen or air in co-current flow and the gasification reactions take place in a dense cloud of very fine particles. Most coals are suitable for this type of gasifier because of the high operating temperatures and the good contact achieved between the coal particles and the gasifying agent.
Cost effective and reliable delivery of coal or any hydrocarbonaceous feedstock to a high pressure gasifier entrained feed reactor is a key process in the overall scheme of gasification. There are primarily three feedstock delivery options for high pressure coal gasifier entrained feed reactors:                1) coal/water slurry        2) coal solids with a carrier gas such as nitrogen or carbon dioxide, and        3) coal/non-aqueous liquid slurry.        
The first two options are widely practiced in commercial operation. The coal/water slurry delivery, though low in cost and reliable, suffers from the disadvantage that it results in low gasification efficiency. The viscosities of the coal/water slurries are also subject to shear rate sensitivity of slurry and there are drawbacks also resulting from the reduction in coal throughput and the handling of sooty water. Alternatively, the coal solids with carrier gas delivery systems result in high gasification efficiency but require expensive lock hoppers and valve systems for safe operation. Lastly, the coal/non-aqueous liquid slurry delivery option has the advantages of an ability to use low cost, high reliability slurry pumps as well as the ability to obtain high gasification efficiencies. However, this last feed option has the drawback of a limited commercial availability of conventionally suitable oils for forming the slurry. An additional source of problems with the present coal/non-aqueous liquid slurry delivery systems is that the rheological properties of the conventional coal-oil slurries, similar to the coal/water slurries, exhibit viscosity sensitivity to shear rate. In general terms, the conventional coal/oil slurries behave as Bingham fluids for which the imposed stress must exceed a critical yield stress to initiate motion. This results in difficulties in pumping and handling in general since behavior as a Newtonian fluid cannot be assumed.
While approaches towards dealing the handling problems with coal/water slurries have been proposed, e.g. in U.S. Pat. No. 6,444,711, the related problems of coal/oil slurries have so far not been adequately addressed.